This invention relates to a heat treating process for the homogenization of nickel-chromium-iron alloy tubing wherein essentially all of the carbon and some of the chromium of the alloy are removed from solid solution to form grain boundary carbide precipitates by a process called sensitization. Continued heating permits chromium to be diffused to restore chromium in the chromium depleted areas associated with the carbide precipitates by a process known as desensitization.
In current practice, it is common to subject finally sized tubing as long as 85 feet in length to a thermal treatment by which mill annealed tubing is placed in an elongate batch type furnace approximately 100 feet in length before it is subjected to a combination sensitization/desensitization isothermal anneal at a temperature of 800.degree. F. to 1400.degree. F. In such a process the carbon is first removed from solid solution in the form of chromium carbide precipitates, while continued heating of the alloy for a protracted period of time permits rediffusion of chromium to produce an alloy that is resistant to acid attack.
A disadvantage of this current practice is that the heat treating time of from 15 to 16 hours at an elevated temperature is prohibitively excessive. Moreover, this process requires an excessively long batch type heat treating facility with a long hot zone to handle the tubing lengths of approximately 85 feet. Such a furnace facility may cost from 3 to 5 million dollars to install and an excessive amount to operate, and is an extremely slow process to carry out.
A typical furnace of the type referred to in this invention heat treats the alloy tubing in any one of its cold worked intermediate stages, before it is cold worked to its final length, thereby permitting the use of a relatively short furnace with a short hot zone, a furnace that is typically existing equipment in most mills.